Flatfish Facts and Indirect Development

Fish like flounders and halibuts are pretty unique, they have both eyes on one side of their body. This allows them to lay flat on the ocean floor and hunt. The interesting thing about this is that these fish aren’t hatched out like this. When first hatched, larval flat fish swim just like any other fish. The eyes of these fish are on either side of their bodies. It isn’t until they undergo metamorphism that the eyes migrate to one side and the fish starts to hang out on the bottom.

For a long time, flatfish seemed to be an exception to evolution. Creationists often used this to “disprove” evolution. This is because no one could find transitional fossils. For a while it seemed that flat fish swam like normal fish and one day they just decided to switch. This changed when someone reexamined previously discovered fossils. Through CT scans it was discovered that these fossils were indeed related to flatfish.

As it turns out flat fish are actually wired to swim flat even when they are just larvae. Fish are able to keep their sense of direction and keep upright by utilizing two organs, their inner ears and their eyes. If their eyes don’t work, they just use their ears. However, flat fish larvae don’t do this. Flat fish have inner ears that are wired for the fish to lay flat, but their eyes are actively keeping them upright. When placed in a pitch black aquarium they are unable to keep upright. They flutter around and swim awkwardly. The video documenting this behavior can be seen by clicking here.  

The reason behind this flatfish behavior is that it prevents the larvae from competing with the adults. If larvae flatfish were on the bottom they would also stand the chance of being consumed by adults. By swimming throughout the water column like a normal fish they are able to better hunt zooplankton.

Source: http://www.pbs.org/wgbh/nova/next/evolution/flatfish-evolution/